1. Field of the Invention
The present invention relates to an inverted merged magnetoresistive (MR) head with a precise track width and more particularly to an inverted merged MR head where a top pole tip portion of a first pole piece defines the track width of the head and a pole tip of a second pole piece is shaped (similar to notching) to confine flux transfer between the pole tips substantially within the defined track width.
2. Description of the Related Art
An inductive write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being located between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Currents are conducted through the coil layer, which produce magnetic fields in the pole pieces. The magnetic fields fringe across the gap at the ABS for the purpose of writing bits of magnetic field information in tracks on moving media, such as in circular tracks on a rotating magnetic disk or longitudinal tracks on a moving magnetic tape.
The second pole piece layer has a pole tip portion which extends from the ABS to a flare point and a yoke portion which extends from the flare point to the back gap. The flare point is where the second pole piece begins to widen (flare) to form the yoke. The placement of the flare point directly affects the magnitude of the magnetic field produced to write information on the recording medium. Since magnetic flux decays as it travels down the length of the narrow second pole tip, shortening the second pole tip will increase the flux reaching the recording media. Therefore, performance can be optimized by aggressively placing the flare point close to the ABS.
Another parameter important in the design of a write head is the location of the zero throat height (ZTH). The zero throat height is the location where the first and second pole pieces first separate from one another after the ABS. ZTH separation is imposed by an insulation layer, typically the first insulation layer in the insulation stack. Flux leakage between the first and second pole pieces is minimized by locating the ZTH as close as possible to the ABS.
Unfortunately, the aforementioned design parameters require a tradeoff in the fabrication of the second pole tip. The second pole tip should be well-defined in order to produce well-defined written tracks on the rotating disk. Poor definition of the second pole tip may result in overwriting of adjacent tracks. A well-defined second pole tip should have parallel planar side walls which are perpendicular to the ABS. In most write heads the second pole tip is formed along with the yoke after the formation of the first insulation layer, the coil layer and the second and third insulation layers. Each insulation layer includes a hard-baked photoresist having a sloping front surface.
After construction, the first, second and third insulation layers present front sloping surfaces which face the ABS. The ZTH defining insulation layer rises from a plane normal to the ABS at an angle (apex angle) to the plane. The sloping surfaces of the hard-baked resist of the insulation layers exhibit a high optical reflectivity. When the second pole tip and yoke are constructed, a thick layer of photoresist is spun on top of the insulation layers and photo patterned to shape the second pole tip, using the conventional photo-lithography technique. In the photo-lithography step, ultraviolet light is directed vertically through slits in an opaque mask, exposing areas of the photoresist which are to be removed by a subsequent development step. One of the areas to be removed is the area where the second pole piece (pole tip and yoke) are to be formed by plating. Unfortunately, when the location of the flare point is placed on the sloping surfaces of the insulation layers, ultraviolet light is reflected forward, toward the ABS, into photoresist areas at the sides of the second pole tip region. After development, the side walls of the photoresist extend outwardly from the intended ultraviolet pattern, causing the pole tip to be poorly formed after plating. This is called "reflective notching". As stated hereinabove this causes overwriting of adjacent tracks on a rotating disk. It should be evident that, if the flare point is recessed far enough into the head, the effect of reflective notching would be reduced or eliminated since it would occur behind the sloping surfaces. However, this solution produces a long second pole tip which quickly reduces the amount of flux reaching the recording medium.
The high profile of the insulation stack causes another problem after the photoresist is spun on a wafer. When the photoresist is spun on a wafer it is substantially planarized across the wafer. The thickness of the resist in the second pole tip region is higher than other regions of the head since the second pole tip is substantially lower on the wafer than the yoke portion of the second pole piece. During the light exposure step the light progressively scatters in the deep photoresist like light in a body of water causing poor resolution during the light exposure step.
A scheme for minimizing the reflective notching and poor resolution problems is to construct the second pole piece of first and second components with the first component forming the second pole tip. The first component is constructed before the insulation layers to eliminate the reflective notching problem. After forming the first pole piece layer and the write gap layer, a photoresist layer is spun on the partially completed head. Ultraviolet light from the photo-patterning step is not reflected forward since the photoresist layer does not cover the insulation stack. Further, the photoresist is significantly thinner in the pole tip region so that significantly less light scattering takes place. After plating the first component the photoresist layer is removed and the first insulation layer, the coil layer and the second and third insulation layers are formed. The second component of the second pole piece is then stitched (connected) to the first component and extends from the ABS to the back gap. Since the second pole tip is well-formed, well-formed notches can be made in the first pole piece, as discussed hereinafter. However, with this head, the ZTH is dependent upon the location of the recessed end of the first component. Since the first component has to be long enough to provide a sufficient stitching area, this length may result in undesirable flux leakage between the first and second pole pieces. Further, the second pole piece component extends to the ABS. Since the second component is typically wider than the first component, as viewed at the ABS, the second pole piece has a T-shape. The upright portion of the T is the front edge of the first component of the second pole piece, and the cross of the T is the front edge of the second component. A problem with this configuration is that during operation, flux fringes from the outer corners of the second component to the first pole piece, causing adjacent tracks to be overwritten.
Once the second pole tip is formed, it is desirable to notch the first pole tip opposite the first and second corners at the base of the second pole tip so that flux transfer between the pole tips does not stray beyond the track width defined by the second pole tip. Notching provides the first pole piece with a track width that substantially matches the track width of the second pole piece. A prior art process for notching the first pole piece entails ion beam milling the gap layer and the first pole piece, employing the second pole tip as a mask. The gap layer is typically alumina and the first and second pole pieces and pole tips are typically Permalloy (NiFe). The alumina mills more slowly than the Permalloy; thus the top of second pole tip and a top surface of the first pole piece are milled more quickly than the gap layer. Further, during ion milling, there is much redeposition (redep) of alumina on surfaces of the workpiece. In order to minimize redep, the milling ion beam is typically directed at an angle to a normal through the layers, which performs milling and cleanup simultaneously. The gap layer in the field remote from the first and second corners of the second pole piece is the first to be milled because of a shadowing effect at the first and second corners caused by the second pole tip when the ion beam is angled. In this case, the ion stream will overmill the first pole piece before the gap layer is removed adjacent the first and second corners of the second pole tip in the region where the notching is to take place. After the gap layer is removed above the sites where the notching is to take place, ion milling continues in order to notch the first pole piece. Overmilling of the first pole piece continues to take place in the field beyond the notches, thereby forming surfaces of the first pole piece that slope downwardly from the notches. As is known, such overmilling of the first pole piece can easily expose leads to the MR sensor, thereby rendering the head inoperative.
Even if overmilling of the first pole piece can be controlled, there is potentially a more troublesome problem, namely overmilling the top of the second pole tip when the unwanted portions of the gap layer are milled and notches are formed. In order to compensate for this overmilling, the aspect ratio (ratio of thickness of photoresist to track width of second pole tip) is increased so that a top portion of the top of the second pole tip can be sacrificed during the milling steps. When the aspect ratio is increased, definition of the second pole tip is degraded because of the thickness of the photoresist, discussed hereinabove, resulting in track overwriting.
In order to minimize overmilling of the first pole piece, another process removes the gap layer, except for a desired portion between the first and second pole tips, by a wet etchant. After the unwanted portions of the gap layer are removed, the first pole piece is ion milled, employing the second pole tip as a mask. The only overmilling of the first pole piece is due to the ion milling of the notches at the first and second corners of the gap layer. This process also eliminates significant redep of the alumina. A problem with this process, however, is that the etching undercuts the gap layer under the base of the second pole tip which is a critical area for the transfer of field signals. The undercut regions provide spaces which can be filled with Permalloy redeposited during subsequent ion milling of the first pole piece or redep of other foreign material upon subsequent milling and clean up steps.
Still another process proposes plating the top and first and second side walls of the second pole tip with a protective metal layer before etching the unwanted portions of the gap layer. When the etching reaches the inside thickness of each protective metal layer, the process is stopped so that the gap layer is not undercut under the base of the second pole tip. It is proposed that the protective metal layer remain in the head because of the difficulty of removing it. Disadvantages of this process are the difficulty of the plating step and the potential of the protective metal layer interfering with the magnetics of the second pole tip.
Another problem with the prior art merged MR head is that the profile of the MR sensor between the first and second gap layers is replicated through the second shield/first pole piece layer to the write gap layer causing the write gap layer to be slightly curved concave toward the MR sensor. When the write head portion of the merged MR head writes data the written data is slightly curved on the written track. When the straight across MR sensor reads this curved data there is progressive signal loss from the center of the data track toward the outer extremities of the data track.
Accordingly, there is a strong-felt need to provide an inductive write head portion of a merged MR head wherein a highly defined track width defining pole tip can be formed without reflective notching and curved written data problems.